Wells drilled for hydrocarbon production are completed with steel casing whose purpose is to control pressure and direct the flow of fluids from the reservoir to the surface. Mechanical integrity of the casing string is important for safety and environmental reasons. The mechanical integrity of a casing string may degrade during the production process due to corrosion which may be induced either mechanically, chemically, or by other means. Regardless of how the corrosion is initiated, or the mechanism by which it progresses, chemical or otherwise, the process removes competent structural material from the casing leaving what remains behind correspondingly thinner and weaker. The state of the mechanical integrity must be estimated or otherwise ascertained by production engineers in order to assess the need for casing repair or replacement prior to catastrophic failure. Several devices for the remote sensing of the casing condition are available. For example, there are casing imaging systems based on acoustical principles. In order to use such systems, casing must be filled with a liquid of constant density whose flow rate is low enough so that the acoustic signals are not lost in noise produced by moving fluids. When conditions favorable for acoustic imaging are not met, mechanical calipers employing numerous tines (50 or 64 have been used) to map casing inner diameter on closely spaced parallel tracks in the casing are used. The mechanical caliper has been suspected of inducing corrosion in some environments.
Various magnetic and electromagnetic techniques have been utilized to detect anomalies in casing, for example, U.S. Pat. Nos. 3,597,678; 4,468,619; and 4,591,785. However, such techniques either have no azimuthal resolution, characterizing the entire casing in terms of a single apparent thickness, or are similar to mechanical calipers in that the azimuthal sampling is limited to a fixed number of sensors. Devices of the latter type are frequently claimed to have 360 degree resolution; however, the actual angular resolution is coarse, being given by the number of sensors on the circumference of the device divided into 360 degrees. For example, twelve such sensors provide an azimuthal resolution of 30.degree.. These devices are not sensitive to the presence of small, localized defects which in time may corrode through the casing, nor can they localize the detected defects to a casing region more precise than a 30.degree. segment of the casing circumference.